With regard to mixture separation, there are various techniques for removing substances (e.g., salts) dissolved in a solvent (e.g., water). Utilization of membrane separation techniques as a process for saving energy and resources is expanding in recent years. Examples of membranes usable in the membrane separation techniques include a microfiltration membrane, ultrafiltration membrane, nanofiltration membrane, and reverse osmosis membrane. These membranes are used in the case of obtaining drinking water from, for example, seawater, brackish water, or water containing a harmful substance, and for producing industrial ultrapure water, wastewater treatments, recovery of valuables, etc.
Most of the currently commercially available reverse osmosis membranes and nanofiltration membranes are composite semipermeable membranes, which are divided into two types: one type has a gel layer and an active layer including a crosslinked polymer on a supporting membrane; and the other type has an active layer formed by polycondensing monomers on a supporting membrane. Among these, a composite semipermeable membrane obtained by coating a supporting membrane with a separation functional layer including a crosslinked polyamide obtained by polycondensation reaction of a polyfunctional amine and a polyfunctional acid halide is widely used as a separation membrane having high permeability and high selectively separating properties (Patent Documents 1 and 2).
A composite semipermeable membrane is configured of a supporting membrane including a substrate and a porous support and a separation functional layer formed on the supporting membrane. A step of producing the composite semipermeable membrane involves winding in which the front surface of the composite semipermeable membrane, which is the surface of the separation functional layer, comes into contact with the back surface of the membrane, which is the surface of the substrate. In case where the porous support excessively infiltrates into the substrate to reach the back surface, which is the surface of the substrate, the porous support that has reached the back surface may stick to the separation functional layer or be rubbed against the separation functional layer to cause damage thereto. Meanwhile, in case where the substrate is made to have an excessively high bulk density in order to prevent the porous support from excessively infiltrating to reach the back surface, which is the surface of the substrate, the porous support does not sufficiently infiltrate into this substrate, resulting in peeling and membrane damage to make it impossible to obtain a satisfactory salt removal ratio.
Patent Document 3 discloses a composite semipermeable membrane having a surface with recesses and protrusions which have a maximum surface level difference of 0.1-1.2 mm, as a sheet-shaped separation membrane in which flow channels can be sufficiently ensured on the membrane surface and the membrane surface sufficiently produces a turbulent effect to render local unevenness in flow less apt to occur.
Patent Document 4 discloses a separation membrane having recesses and protrusions and a separation membrane element which are effective in improving the separation/removal performance, improving separation membrane performances, such as increasing the permeation flow rate per unit time, and improving the resistance to chemicals such as acids and alkalis.